Method of deoxidizing stainless steel and electropolishing of molybdenum

ABSTRACT

A METHOD OF ELECTROPOLISHING WORKPIECES AS MOLYBDENUM ELECTRODES AND GRIDS AND REMOVAL OF OXIDIATION ON STAINLESS STEEL, WITH A CURRENT DENSITY ON THE ORDER OF 0.9 TO 5 AMPERES PER SQ. INCH IN A VOLTAGE RANGE IN THE ORDER OF 15 TO 90 VOLTS, USING AN ANHYDROUS ACETIC ACID BASED BATH CONTAINING ABOUT 10 PERCENT SULFURIC ACID. THE PROCESS IS RELATIVELY FREE OF VARIOUS HAZARDS COMMON TO THE ELECTROPOLISHING OF METALS, E.G. HIGH CELL VOLTAGE, EXCESSIVE METAL LOSS AND DANGEROUS ELECTROLYTE COMPOSITIONS.

United States Patent 3,751,352 METHOD OF DEOXIDIZING STAINLESS STEEL AND ELECTROPOLISHING 0F MOLYBDENUM George R. Blair, Culver City, and Sherman R. Keller, Valinda, Calif., assignors to Hughes Aircraft Company, Culver City, Calif.

No Drawing. Application June 22, 1970, Ser. No. 48,551, which is a continuation-in-part of abandoned application Ser. No. 721,153, Apr. 15, 1968. Divided and this application Mar. 2, 1972, Ser. No. 282,197

Int. Cl. C23h 3/00 US. Cl. 204-145 R 4 Claims ABSTRACT OF THE DISCLOSURE A method of electropolishing workpieces as molybdenum electrodes and grids and removal of oxidation on stainless steel, with a current density on the order of 0.9 to 5 amperes per sq. inch in a voltage range in the order of to 90 volts, using an anhydrous acetic acid based bath containing about 10 percent sulfuric acid. The process is relatively free of various hazards common to the electropolishing of metals, e.g. high cell voltage, excessive metal loss and dangerous electrolyte compositions.

This application is a division of application Ser. No. 48,551 filed June 22, 1970, now US. Pat. No. 3,679,559. Said earlier application was a continuation-in-part of application Ser. No. 721,153, filed Apr. 15, 1968, now abandoned.

BACKGROUND OF THE INVENTION The polishing of metals mechanically and electrolytically has been known for many years; Brimi, M.A., Electrofinishing American Elseiver Publishing Co. Inc. (1965); Cortes, F. R. Electrolytic Polishing of Refractory Metals, Metal Progress 80 No. 2, 97-100 1961) and Tegart, W. 1., The Electrolytic and Chemical Polishing of Metals in Research and Industry Pergamon Press (1959).

In the electrochemical field combinations of hydrofiuoric and other acids, create mechanical and physical hazards and are characterized by high cell voltage and current density coupled with excessive metal loss. This has created a desire for less hazardous treating compositions in conjunction with improved control of metal loss, particularly in the elfecting smoothness and polishing of molybdenum electrodes and grids or molybdenum helixii, and deoxidizing stainless steel.

SUMMARY OF THE INVENTION This invention relates to an improvement in a method of deoxidizing stainless steel and electropolishing molybdenum metal workpieces, and especially molybdenum and molybdenum alloy anodes and grids, compositions therefor, and the products thereof. More particularly, the invention relates to an improvement in the electrolytic method of deoxidizing stainless steel or electropolishing of molybdenum electrodes .and grids, more economically convenient compositions therefor and the products thereof.

Accordingly, it is an object of this invention to provide the art with an improvement overcoming the above problems and the improved metal articles produced thereby.

It is a more particular object of this invention to provide a more economical and convenient method for improving the electropolishing and improving electrical characteristics of molybdenum electrodes and grids or helices, providing compositions therefor and the products thereof.

It is a further object of this invention to provide an improvement in the deoxidizing of other metal objects and particularly objects of stainless steel.

3,751,352 Patented Aug. 7, 1973 ice DESCRIPTION OF THIS INVENTION ILLUSTRATED BY PREFERRED EMBODIMENTS or about 10 ml. concentrated sulfuric acid to about 100 ml. glacial acetic acid, as the active polishing components.

The metal workpiece (a molybdenum electrode), treated in the above composition, was first thoroughly cleaned with a vapor degreaser, or Freon, or both. It is imperative that the parts to be deoxidized or polished be clean and free of oil and wax. Repeated cooling and dipping into a solvent therefor as trichloroethylene vapors effectively removes the oil and wax.

The cleaned metal piece, as the molybdenum electrode, was then positioned in the bath, as the anode. The molybdenum electrode is preferably supported by molybdenum clips or rods to avoid contaminants. Otherwise, stainless steel, copper, brass, and the like support rod or clip means may be used with the appropriate metal and preferably masked with a conventional dip-type maskant. The small molybdenum electrode (as the anode), positioned facing the cathode, was electropolished with a 1 ampere current flow at 17 volts in 3 minutes time. Under these conditions, the rate of metal removal for the surface being polished is 0.05 mil/minute. This rate of removal, in the relative time, is much smaller than for any other baths known to the art. Small electrodes are polished without stirring, but mild agitation of the large anodes appears to be necessary and helps obtain uniform polish. In the present processing, a current density of 0.9 amp. per sq. in. is being utilized with the voltage pushed up to volts. With this increased voltage at a current density, on the order of 0.9 amp. per sq. in., the molybdenum metal loss is not observably or materially changed. Thus, in the overall the current density may controllably range from about 0.9 ampere to about 5 amperes per sq. in. with the voltage correspondingly regulated and controlled between the order of 15 to 90 volts. For best results the lower current density is utilized with the higher voltage and the greater current density with the lower voltage, as the operators choice for best results with the metal workpiece being treated in the solution.

After polishing and rinsing in preparation for use, the resultant electrode product was discovered to have improved electrical properties, as notably reduced high voltage corona. Also, such polishing prevented arcing between high voltage elements and grounded elements, thus enabling greater than normal voltage to be impressed on such treated electrodes.

Other tests with this solution illustrated that low current densities and higher voltages or higher current density and low voltage yielded excellent polishes in a few minutes with metal removal being very small in a few minutes process. Also, the solution did not heat up to any excessive temperature. It is also useful for polishing inside a small hole, and obtaining a much smoother polish than possible by hand.

While some variations may be possible in solution concentrations, temperature and time, the polishing function is a non-aqueous medium and while the effect of temperatures from 18 C. to 100 C. is not very great, other than a more rapid metal removal at elevated temperatures, there is little to be gained by raising the bath temperature unless etching results are desired in hot solutions. Excellent polishing is obtained with the anhydrous bath at room temperature and and there is practically no temperature build-up in the bath during the polishing operation. With the acetic acid bath, no cooling is required. In continuous operation, the temperature raise may be slight to about 30 C. This may be overcome by cooling, for continuous operation, if desired. The voltage range required is only 17 to 30 volts up to the limit of 40 volts, or lower voltage on the grids. However, it is now discovered that the voltage can be pushed up to about 90 volts with a current density of 0.9 amp. without further observable or appreciable loss of molybdenum in the anode work piece. Below 12 volts, little polishing occurred, if any. At 15 volts, excellent polishes were obtained up to the limit of 40 volts on grids and up to 90 volts on the anodes. At higher current values and voltages, the rate of metal removal and etching of molybdenum becomes excessive. Herein, excellent finishes are provided with metal removal of from 0.1 mil to 0.15 mil in 3 minutes in the bath with low current densities and of about 40 to 90 volts.

The voltage and current are coupled and dependent on the cell resistance. The bath employed requires about 1 ampere/ 6 cm. (1 ampere/m As indicated in recent development, it has been discovered that rapid polishing can be accomplished by controlling the amperes and raising the voltage, as indicated. During the polishing or deoxidizing process, the current normally reaches a maximum after a few seconds, and then drops to a steady value.

Molybdenum electrodes generally do not require more than 3-4 minutes polishing time in the method herein provided. At the volt-ampere/in. setting of about 20, the bath will remove 0.05 mil per minute. For small focus electrodes, the voltage should be about 17 and the current will stabilize at about 1.5 amperes. For large anodes, 30 volts should be used and the current will be 4-5 amperes at completion of the time period. As recently discovered, the voltage may be raised to 90 volts and with low ampere of current density on the order of 0.9, the metal loss of the molybdenum is not excessive or detrimental. Preferably, upon completion, the voltage is turned off with sample in the bath and the workpiece removed and cleaned. Rinsing with tap Water, distilled water, and dipping in an alkaline bath, as ammonium hydroxide, may be used. After the final rinse (preferably distilled water) the polished metal is preferably rinsed with methanol dried and packaged, or ready for use.

As indicated, the anode is placed in the bath facing the cathode and each workpiece generally dictates its own configuration of the cathode. For example, if large or deep holes are present, they should be equipped with a separate, or auxiliary, cathode in the center, and large anodes may require a ring cathode placed centrally and in close proximity to the surface of the workpiece.

As described, the anhydrous composition of sulfuric and acetic acid is preferred for the electropolishing of molybdenum electrodes at relatively low current densities and low voltage of not over about 90 volts with improved control of metal removal, without obvious or detrimental pitting and in the order of 0.1 mil for about a three-minute process.

For uniform and improved results, the polishing operation is better effected in clean bath compositions and treating baths from which an excess of sludge has been removed by replacing an equivalent amount of new solution for the removed sludge portion.

In general, in the anhydrous composition described of essentially acetic acid and sulfuric acid, the polishing action requires the formation of a viscous film on the surface of the workpiece being polished and this depends primarily on the solution viscosity. Other metals, for example, aluminum requires agitation for proper electropolishing, copper requires no agitation and, in fact, agitation leads to striations and uneven surface finish. When the anhydrous acetic acid bath is first prepared and a metal work piece immersed, the nature of the viscous layer may be readily observed. It is colored blue and drips off the work piece or flows in a laminar fashion at about the speed of free-flowing molasses. Agitation, effecting removal of the viscous layer, causes etching and severe striation formation. Consequently, no bath agitation per se is used with the acetic-sulfuric-acid bath, provided herein.

It has been discovered, however, that mild agitation of the workpiece is beneficial on large (3 /2" diameter) parts. Without such movement thereof, etching occurs. This etching appears to be related to the thickness of the viscous layer and the low current which accompanies it. Smaller samples however do not require movement because the viscous layer thins itself out fast enough to affect proper current flow. That is, on a large work area, the viscous layer cannot be dissipated so that the current falls below an optimum value and etching occurs without agitation of the workpiece.

It has thus been discovered that there is herewith provided an electropolishing bath of an anhydrous acid combination for polishing a metal surface and particularly molybdenum anodes were accomplished in 3-5 minutes polishing time at low current densities of about 1.5 to 4-5 amperes and low voltage of 15 to 40 volts and preferably in the order of 17 to 30 volts. Work samples polished in 3-4 minutes, as small molybdenum focus electrodes, were previously maintained at a voltage of about 17 and the current stabilized at about 1.5 amperes. For larger anodes, 30 volts were used and the current was stabilized at 4-5 amperes at the completion of the timed period. In generalization, for reproducible polishing, the volt-amperes/ square inch should be in the region of 20 with the higher ampere range. In the lower ampere range of about 0.9, the voltage can be pushed up to about for comparable results. For example, as indicated, the small focus electrodes have a v.-a./in. of about 17 and the large anode has a value of about 23 for the bath to remove about 0.05 mil from the workpiece surface a minute, or about 1.5 mils in 3 to 4 minutes.

As indicated, it has now been discovered that the polishing effect can be realized with improved results on new or used electrodes in the lower current range and higher voltage, in utilizing the above anhydrous solution. In addition, the solution serves as an excellent medium for removal of oxidation on stainless steel, without appearing to remove any metal, when oxidized stainless steel objects are suspended in the solution and treated therein in a like manner.

That is, an oxidized stainless steel workpiece, as an electrode or tableware, when suspended and treated in a like manner, as the molybdenum workpiece, is deoxidized without any observable or readily determinable loss of metal. The particular current density and voltage utilized with respect to time is under control of the operator and results desired.

Having described the present embodiments of our discovery in accordance with the patent statutes, it will now be apparent that some modifications and variations may be made without departing from the spirit and scope thereof. The specific embodiments described are provided by way of illustration and are illustrative of our discovery, invention or improvements which are to be limited only by the terms of the appended claims.

What is claimed is:

1. The method of deoxidizing a stainless steel metal workpiece surface comprising steps of:

(a) preparing an anhydrous acid bath in which the active deoxidizing constituents are about 90 parts acetic acid to about parts concentrated sulfuric acid;

(b) supporting said stainless steel workpiece to be deoxidized as an anode workpiece in said bath;

(0) positioning a cathode in facing relationship to the surface of the said stainless steel workpiece;

(d) establishing a controlled voltage of about to about 90 volts and a controlled current of about 0.9 ampere to about 5 amperes on said stainless steel workpiece; and

(e) affecting removal of oxidation from said stainless steel with substantially no removal of metal from said stainless steel.

2. The method of claim 1 wherein the voltage is of about to about volts.

6 3. The method of claim 1 wherein the workpiece is supported in a stationary position during deoxidation.

4. The method of claim 1 wherein the workpiece is agitated in said bath.

References Cited UNITED STATES PATENTS 1,334,092 3/1920 Harmeling 204445 2,315,568 4/1943 Weverlund 204- 10 3,002,899 10/1961 Reid 204-32 FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R. 

